Package-on-Package Structures and Methods for Forming the Same

ABSTRACT

A package includes a package component, which further includes a top surface and a metal pad at the top surface of the package component. The package further includes a non-reflowable electrical connector over and bonded to the metal pad, and a molding material over the package component. The non-reflowable electrical connector is molded in the molding material and in contact with the molding material. The non-reflowable electrical connector has a top surface lower than a top surface of the molding compound.

This application claims the benefit of U.S. Provisional Application No.61/783,050, filed on Mar. 14, 2013, and entitled “Package-on-PackageStructures and Methods for Forming the Same,” which application ishereby incorporated herein by reference.

BACKGROUND

In a conventional Package-on-Package (PoP) process, a top package, inwhich a first device die is bonded, is further bonded to a bottompackage through solder balls. The bottom package may also include asecond device die bonded therein. The second device die may be on thesame side of the bottom package as the solder balls.

Before the bonding of the top package to the bottom package, a moldingcompound is applied on the bottom package, with the molding compoundcovering the second device die and the solder balls. Since the solderballs are buried in the molding compound, a laser ablation or drillingis performed to form holes in the molding compound, so that the solderballs are exposed. The top package and the bottom package may then bebonded through the solder balls in the bottom package.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 6B are cross-sectional views of intermediate stages inthe manufacturing of a package-on-package structure in accordance withsome exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A Package-on-Package (PoP) structure and the method of forming the sameare provided in accordance with various embodiments. The intermediatestages of forming the PoP structure are illustrated in accordance withsome embodiments. The variations of the embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

Referring to FIG. 1, package component 10 is provided. In someembodiments, package component 10 is package substrate, and hencethroughout the description, package component 10 is referred to aspackage substrate 10, although it can be of other types. In alternativeembodiments, package component 10 comprises an interposer. Packagecomponent 10 may be a part of a package component that includes aplurality of identical package components 10. For example, packagecomponent 10 may be a package substrate, and is located in an un-sawedpackage substrate strip that comprises a plurality of package substratesformed as an array.

Package substrate 10 may comprise substrate 18 that is formed of adielectric material. Alternatively, substrate 18 may be formed of othermaterials such as a semiconductor material, for example, silicon. Insome exemplary embodiments, substrate 18 is a build-up substrate that isbuilt on a core, as shown in FIG. 1. Alternatively, substrate 18 may bea laminate substrate which includes laminated dielectric films adheredtogether through lamination. When substrate 18 is formed of a dielectricmaterial, the dielectric material may comprise composite materials thatare mixed with glass fiber and/or resin.

Package substrate 10 is configured to electrically couple electricalconnectors 12 on first surface 10A to conductive features 16 on secondsurface 10B, wherein surfaces 10A and 10B are opposite surfaces ofpackage substrate 10. Electrical connectors 12 and conductive features16 may be metal pads, for example, and hence are referred to as metalpads 12 and 16, respectively. Package substrate 10 may includeconductive connections such as metal lines/vias 14, which may furthercomprise through-vias penetrating through substrate 18.

In some embodiments, solder paste 22 is coated on some or all metal pads12. Next, as shown in FIG. 2, electrical connectors 24 are bonded onmetal pads 12. The bonding may include placing electrical connectors 24on solder paste 22, and heating package substrate 10, so that electricalconnectors 24 are secured on metal pads 12. Electrical connectors 24 areelectrically connected to metal pads 12, wherein solder paste 22provides the securing force to fix electrical connectors 24. Inaddition, solder paste 22 helps in providing an electrical connectionbetween electrical connectors 24 to the underlying metal pads 12.

In some embodiments, electrical connectors 24 are non-reflowable metalballs. Electrical connectors 24 do not melt under the temperatures thatare typically used for melting solders, which temperatures may be in therange of about 200° C. to about 280° C., for example. In accordance withsome embodiments, electrical connectors 24 are copper balls, aluminumballs, or the like. Electrical connectors 24 may also include metalsselected from the group consisting essentially of copper, aluminum,nickel, platinum, tin, tungsten, and alloys thereof. The shapes ofelectrical connectors 24 may be near-perfect ball shapes in someembodiments, except that the formation of balls may have processvariations. After the bonding of electrical connectors 24, electricalconnectors 24 may remain to have a ball shape, with the bottom end ofeach of the balls 24 contacting the respective underlying metal pad 12through a point, although the balls may also be spaced apart from theunderlying metal pads 12 by a thin layer of solder paste 22. The bottomsurface (which is a part of a near-perfect surface of a ball) of each ofelectrical connectors 24 is also electrically coupled to the respectiveunderlying metal pad 12 through solder paste 22, and also possiblythrough the contacting point.

In alternative embodiments, electrical connectors 24 are solder balls,which may comprise either eutectic solder or non-eutectic solder. Whenformed of solder, the bonding of electrical connectors 24 includes areflow process, wherein the solder-containing electrical connectors 24are reflowed. In these embodiments, solder paste 22 may not be used.Instead, a flux (not shown) may be applied to improve the reflow.

Referring to FIG. 3, package component 20 is bonded to package substrate10 through metal pads 12. Hence, bottom package 100, which includespackage substrate 10 and die 20, is formed. Package component 20 may bea device die, and hence is alternatively referred to as die 20hereinafter, although it may also be another type of package componentsuch as a package. Dies 20 may be a circuit die comprising integratedcircuit device (not shown) such as transistors, capacitors, inductors,resistors, and/or the like. Furthermore, die 20 may be a logic circuitdie such as a Central Computing Unit (CPU) die. The bonding of die 20 tometal pads 12 may be achieved through solder bonding or directmetal-to-metal bonding (such as copper-to-copper bonding).

Referring to FIG. 4, molding material 28 is molded on die 20 and packagesubstrate 10. A curing process is performed after molding material 28 isapplied. Molding material 28 may include a filler, a polymer, and ahardener in accordance with some embodiments. In an exemplaryembodiment, the polymer may be a molding compound, an underfill, aMolding Underfill (MUF), an epoxy, or the like. In some embodiments,molding material 28 has its top surface 28A level with the top surface20A of die 20, and hence die 20 is exposed through molding material 28.In alternative embodiments, molding material 28 may fully encapsulatedie 20 therein, with a portion of molding material 28 being overlappingdie 20. In yet alternative embodiments, molding material 28 may encircleand be in contact with the lower part of die 20, while the upper part ofdie 20 is over top surface 28A of molding material 28.

The top portions of electrical connectors 24 are over top surface 28A ofmolding material 28. In some exemplary embodiments, height H1 of theupper portions of electrical connectors 24 that are over top surface 28Ais greater than about ¼, or greater than about ⅓, of height H2 ofelectrical connectors 24. Ratio H1/H2 may also be close to 0.5 in someembodiments.

Referring to FIG. 5, the top portions of electrical connectors 24 areremoved. The bottom portions of electrical connectors 24 remainun-removed, and hence electrical connectors 24 become partial balls. Insome embodiments, the removal of electrical connectors 24 is performedby generating electrical arc 30 between electrical connectors 24 andelectrodes 32. The temperature of electrical arc 30 may cause theexposed surface portions of electrical connectors 24 to rise to a veryhigh temperature, for example, between about 8,000° C. and about 12,000°C., and hence the surface portions of electrical connectors 24 areevaporated. In accordance with some embodiments, to generate electricalarc 30, a plurality of electrodes 32, which are formed of metal, areplaced over, and are aligned to electrical connectors 24. Each of theelectrical connectors 24 that is intended to have it top portion removedis aligned to one of the electrodes 32. Voltage Varc is applied betweenelectrodes 32 and electrical connectors 24 in order to generateelectrical arc 30, wherein voltage source 36 is electrically coupled toelectrodes 32 and electrical connectors 24 to provide voltage Varctherebetween. Voltage Varc may range from, for example, about 50 V toabout 500 V, although different voltages may be used.

In some embodiments, electrical connectors 24 are electrically grounded,for example, through bottom metal pads 16 and bottom electrode 34, whichelectrically connect metal pads 16 to voltage source 36. Since metalpads 16 are electrically connected to electrical connectors 24,connectors 24 are electrically coupled to voltage source 36. During thegeneration of electric arc 30, electrodes 32 may be controlled toapproach electrical connectors 24. The electrical fields betweenelectrodes 32 and electrical connectors 24 accordingly rise, andeventually electrical arc 30 occurs when the distances betweenelectrodes 32 and electrical connectors 24 are short enough. After thetop portions of electrical connectors 24 are removed, electrodes 32, 34,and voltage source 36 are taken away.

In some embodiments, after the top portions of electrical connectors 24are removed, the resulting top surfaces 24A of electrical connectors 24are non-planar. For example, top surfaces 24A may be rounded, with thelowest points close to the centers of the respective top surfaces 24A.Furthermore, at least parts of top surfaces 24A may be lower than topsurface 28A of molding material 28. In some embodiments, the entiretiesof electrical connectors 24 are level with or lower than top surface 28Aof molding material 28, which means that the entities of top surfaces24A are level with or lower than top surface 28A of molding material 28.

FIGS. 6A and 6B illustrate the bonding of top package 50 to bottompackage 100. The resulting structure is hence a PoP structure. Forexample, referring to FIG. 6A, top package 50 is first placed overpackage substrate 10. Top package 50 may be a package that includesdevice die 52 and package substrate 54, wherein device die 52 is bondedto package substrate 54. In some exemplary embodiments, device die 52 isa memory die such as a Static Random Access Memory (SRAM) die, a DynamicRandom Access Memory (DRAM) die, or the like. Furthermore, moldingmaterial 56 may be pre-molded on device dies 52 and package substrate 54before the bonding of top package 50 to package substrate 10.

In some embodiments, top package 50 includes electrical connectors 58 atthe bottom surface of top package 50. The positions of electricalconnectors 58 are aligned to the positions of electrical connectors 24.Since electrical connectors 24 are recessed below the top surface 28A ofmolding material 28, electrical connectors 58 may be settled in therecesses easily, and hence top package 50 does not slide with relativeto bottom package 100. Furthermore, since electrical connectors 24 arerecessed, the total thickness of the resulting PoP structure is reduced.In some embodiments, electrical connectors 58 comprise solder, and maybe solder balls. In alternative embodiments, electrical connectors 58may comprise non-reflowable portions such as metal pillar or a metalpad. The non-reflowable portions of electrical connectors 58 may beformed of a material selected from the same candidate materials ofnon-reflowable electrical connectors 24. Furthermore, electricalconnectors 58 may comprises solder caps on the surface of thenon-reflowable portion.

Referring to FIG. 6A, in the embodiments in which electrical connectors24 are non-reflowable, the solder in electrical connectors 58 arereflowed, during which time electrical connectors 24 are not reflowed.The resulting electrical connectors 58 bond electrical connectors 24 totop package 50. After the reflow, electrical connectors 58 have thebottom surfaces fitting the shapes and the profiles of the top surfacesof electrical connectors 24.

Referring to FIG. 6B, in the embodiments in which electrical connectors24 comprise solder, after a reflow process, the solder in solder balls58 is merged with the solder in electrical connectors 24 to form solderregions 60, which join top package 50 to package substrate 10.

In the embodiments of the present disclosure, by removing the topportions of the electrical connectors in the bottom package, the overallheight of the PoP structure is reduced. Furthermore, non-reflowablemetal balls are used in the bottom package. As a result, the pitch ofthe electrical connectors bonding the top package to the bottom packagemay be reduced since the sizes and the shapes of the non-reflowablemetal balls are easy to control.

In accordance with some embodiments, a package includes a packagecomponent, which further includes a top surface and a metal pad at thetop surface of the package component. The package further includes anon-reflowable electrical connector over and bonded to the metal pad,and a molding material over the package component. The non-reflowableelectrical connector is molded in the molding material and in contactwith the molding material. The non-reflowable electrical connector has atop surface lower than a top surface of the molding compound.

In accordance with other embodiments, a package includes a packagesubstrate comprising a top surface, and a metal pad at the top surfaceof the package substrate. A partial metal ball is disposed over andbonded to the metal pad, wherein the partial metal ball is formed of anon-reflowable material. A die is over and bonded to the packagesubstrate. A molding material is disposed over the package substrate,wherein the partial metal ball and a lower portion of the die are moldedin the molding material.

In accordance with yet other embodiments, a method includes molding amolding material over a package substrate to form a bottom package,wherein an electrical connector of the bottom package is exposed througha top surface of the molding material. A top portion of the electricalconnector is then removed using an electrical arc, wherein a bottomportion of the electrical connector remains after the step of removing.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A package comprising: a bottom packagecomprising: a first package component comprising: a top surface; and ametal pad at the top surface of the first package component; anon-reflowable electrical connector over and bonded to the metal pad;and a molding material over the first package component, with thenon-reflowable electrical connector partially molded in the moldingmaterial and in contact with the molding material, wherein thenon-reflowable electrical connector has a top surface lower than a topsurface of the molding material; a top package over the bottom package;and a solder region bonding the non-reflowable electrical connector tothe top package, wherein the solder region and the non-reflowableelectrical connector comprise different materials.
 2. The package ofclaim 1, wherein the top surface of the non-reflowable electricalconnector comprises a rounded portion, with a center portion of therounded portion being lower than remaining portions of the top surface.3. The package of claim 1, wherein the non-reflowable electricalconnector has a rounded bottom surface, with the bottom surface having ashape of a part of a ball.
 4. The package of claim 3 further comprisinga solder paste disposed between, and in contact with, the rounded bottomsurface of the non-reflowable electrical connector.
 5. The package ofclaim 1, wherein the non-reflowable electrical connector is in physicalcontact with the solder region.
 6. The package of claim 1, wherein thenon-reflowable electrical connector comprises copper.
 7. The package ofclaim 1 further comprising a second package component bonded to the topsurface of the first package component, wherein the molding materialmolds at least a lower part of the second package component therein. 8.A package comprising: a bottom package comprising: a package substratecomprising: a top surface; and a metal pad at the top surface of thepackage substrate; a partial metal ball over and bonded to the metalpad, wherein the partial metal ball is formed of a non-reflowablematerial; a die over and bonded to the package substrate; and a moldingmaterial over the package substrate, wherein the partial metal ball anda lower portion of the die are molded in the molding material; a toppackage over the bottom package; and an electrical connector bonding thepartial metal ball to the top package, wherein the electrical connectorand the partial metal ball comprise a same conductive material.
 9. Thepackage of claim 8, wherein the partial metal ball has a top surface,and wherein a center portion of the top surface of the partial metalball is lower than a top surface of the molding material.
 10. Thepackage of claim 8, wherein the partial metal ball has a non-planar topsurface.
 11. The package of claim 10, wherein the top surface of thepartial metal ball comprises a rounded portion, with lowest point of therounded portion being close to a center of the top surface of thepartial metal ball.
 12. The package of claim 8 further comprising asolder paste disposed between, and in contact with, a rounded bottomsurface of the partial metal ball and the metal pad.
 13. The package ofclaim 8, wherein the partial metal ball and the electrical connectorcomprise copper.
 14. The package of claim 8, wherein the partial metalball and the electrical connector comprise solder.
 15. A methodcomprising: molding a molding material over a package substrate to forma bottom package, wherein an electrical connector of the bottom packageis exposed through a top surface of the molding material; and removing atop portion of the electrical connector using an electrical arc, whereina bottom portion of the electrical connector remains after the step ofremoving.
 16. The method of claim 15 further comprising bonding a toppackage component to the package substrate, wherein the top packagecomponent is bonded to the package substrate through the electricalconnector.
 17. The method of claim 15, wherein the step of removing thetop portion of the electrical connector is performed until a portion ofa top surface of the electrical connector is lower than the top surfaceof the molding material.
 18. The method of claim 15, wherein theelectrical connector comprises a solder region, and wherein the methodfurther comprises: placing a solder ball over a metal pad of the packagesubstrate; and before the step of molding, reflowing the solder ball toform the electrical connector.
 19. The method of claim 15, wherein afterthe step of removing the top portion of the electrical connector, anentirety of a top surface of a remaining portion of the electricalconnector is lower than the top surface of the molding material.
 20. Themethod of claim 15, wherein the step of generating the electrical arccomprises: placing an electrode over and aligned to the electricalconnector; and applying a voltage difference between the electricalconnector and the electrode.